Various communication systems are known in the art. Pursuant to many such systems, an information signal is modulated on to a carrier signal and transmitted from a first location to a second location. At the second location, the information signal is demodulated and recovered.
Typically, the communication path used by such a system has various limitations, such as bandwidth. As a result, there are upper practical limitations that restrict the quantity of information that can be supported by the communication path over a given period of time. Various modulation schemes have been proposed that effectively increase the information handling capacity of the communication path as measured against other modulation techniques. Sixteen-point quadrature amplitude modulation (QAM) provides a constellation of modulation values (distinguished from one another by each having a different combination of phase and amplitude) wherein each constellation point represents a plurality of information bits.
By virtue of their changing amplitude from QAM-symbol time-to-QAM-symbol time, QAM symbols in a QAM communication system require linear power amplification to be able to accurately distinguish one QAM symbol at one amplitude level and another QAM symbol at some other power level. In a radio communications system, QAM symbols require a very linear amplification prior to broadcasting them on an antenna. In QAM systems, non-linear amplification of QAM symbols in a QAM signal, (which QAM signal is typically considered to be a pulse-shape filtered and frequency up-converted stream of QAM symbols), in a radio transmitter can make coherent demodulation impossible. Another more common problem with using non-linear amplifiers with QAM modulation is the frequency splatter caused by non-linear amplification of a signal. For this reason, linear power amplifiers are required in QAM radio transmitters, which power amplifiers increase in cost, size, and complexity as their output power level and/or linearity increase.
A problem in the design of a linear power amplifier is providing the ability of an amplifier to accommodate widely fluctuating input power levels while producing at its output a faithful reproduction of the input signal. While an amplifier can be readily designed to have a linear power amplification of a relatively constant-amplitude input signal, designing an amplifier that can accommodate a peak power level that might, at any given time, exceed the average power level by several decibels (db) can significantly increase the cost and size of the amplifier.
In QAM communications systems, the ratio of the peak power level to average power level of a QAM symbol stream will usually continuously vary by virtue of the fact that the data represented by the QAM symbols itself varies randomly. Accordingly, power amplifiers for QAM communications systems must be capable of handling a significant peak to average power level ratio and, accordingly, any reduction in the peak to average power ratio eases the requirements of a QAM power amplifier.
Some prior art, single channel QAM systems are frequently transmitted on a communications channel, such as a radio frequency channel, in conjunction with a pilot component. Such pilot components, by constructively or destructively adding with other QAM symbols can at times aggravate the the peak to average power level ratio requirements of a QAM power amplifier, thereby further aggravating the requirements of such an amplifier.
Any methodology by which the ratio of peak power amplitude to average power amplitude is reduced would therefore simplify and reduce the amplifier cost associated with a QAM system and would be an improvement over the prior art.